Malloc already includes a length and most arrays are heap based. I wish it could be exposed in a nice way. Of course it would have to support sub allocators or it wouldn't be C.
There's malloc_usable_size[1], assuming you mean asking the memory allocator what the array size is. But chances are that wouldn't work correctly, because what the amount of memory a malloc calls gives you and the amount you requested are often not the same. Modern memory allocators round up the request size to the nearest "size class".
